"A METHOD AND APPARATUS FOR ADAPTIVELY CHANGING UPLINK POWER CONTROL SCHEME ACCORDING TO MOBILE STATUS IN A TDD MOBILE COMMUNICATION SYSTEM"

Abstract

An apparatus and method for changing an uplink power control scheme according to mobile status in a TDD mobile communication system are provided. A subscriber station transmits to a base station a power control change request message including information about a requested power control scheme. Upon receipt of the power control change request message, the base station selects a power control scheme for the uplink of the subscriber station and transmits to the subscriber station a power control change command message including information about the selected power control scheme. The subscriber station extracts, upon receipt of the power control change command message from the base station, the power control scheme information from the power control change command message and selects a power control scheme according to the extracted information.

Full Text

Description
APPARATUS AND METHOD FOR ADAPTIVELY CHANGING
UPLINK POWER CONTROL SCHEME ACCORDING TO
MOBILE STATUS IN A TDD MOBILE COMMUNICATION
SYSTEM
Technical Field
[ 1 ] The present invention relates generally to an apparatus and method for detennining
a power control scheme in a time division duplex (TDD) mobile communication
system, and in particular, to an apparatus and method for changing an uplink power
control scheme according to the status of a subscriber station (SS).
Background Art
[2] As one of the duplex schemes, Time Division Duplex (TDD) uses two distinct sets
of time slots on the same frequency for the uplink from a base station (BS) to a
Subscriber Station (SS) and the downlink from the SS to the BS. Another major duplex
scheme is frequency division duplex (FDD). FDD uses two distinct frequencies for the
uplink and the downlink.
[3] Unlike FDD, the uplink and the downlink share the same frequency band in TDD
and are separated by time slots dedicated to them. That is, time slots are separately
preset for the uplink signal and the downlink signal. Therefore, the uplink and
downlink signals are transmitted only in their assigned time slots. TDD has the
advantage of high frequency use efficiency.
[4] The mobile communication system schedules bursty uplink/downlink packets. Par-
ticularly, the BS decides a modulation and coding scheme (MCS) for the resources to
be allocated and already allocated resources in uplink/downlink packet scheduling for
an SS. An MCS level to be used depends on the status of the SS. For the uplink
scheduling, the BS takes into account the maximum transmit power of the SS. Since
the transmit power of the SS is restricted to a set level, the BS performs scheduling
taking into account the allocated resources, an MCS level to be applied for the
resources, and the transmit power limit of the SS. To do so, the scheduler of the BS
must have knowledge of the power headroom or transmit power of the SS.
[5] Typically, the mobile communication system uses downlink and uplink power
control to increase call capacity and achieve good call quality. That is, if the BS
receives a signal from an SS at a signal-to-interference ratio (SIR) that ensures the
minimum required call quality by controlling the transmit power of all of the SSs,
system capacity can be maximized. In the case where the signal from the SS is
received in the BS at a higher power level, the performance of the SS is increased at

the expense of increasing interference from other SSs sharing the same channel. As a
result, system capacity is decreased or the call quality of other subscribers drops.
[6] Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency
Division Multiple Access (OFDMA) has recently been proposed as a physical layer
scheme for a 4th generation mobile communication system. The above-described power
control has also emerged as a challenging issue to the OFDM/OFDMA system.
[7] OFDM/OFDMA is a transmission scheme based on the IEEE 802.16 standard, in
which a serial modulation symbol sequence is transmitted as parallel data. OFDM/
OFDMA operates in TDD. In OFDM, 256 modulation symbols are Fast-
Fourier-Transformed (FFT-processed) to one OFDM symbol, whereas in OFDMA,
one OFDM symbol is formed with more modulation symbols. According to the IEEE
802.16-based OFDMA, the subcarriers of one OFDM symbol are grouped into
subchannels and a plurality of OFDM symbols form one frame.
[8] FIG. 1 illustrates an OFDMA frame structure specified by IEEE 802.16. The
horizontal axis represents OFDM symbol indexes and the vertical axis represents
subchannel indexes.
[9] Referring to FIG. 1, an OFDMA frame is comprised of a plurality of bursts each
marked by a square on a time-frequency plane. Since the frame is time-
division-duplexed, the downlink period and the uplink period can be flexibly
controlled. For example, kth through (k+8)th symbols are allocated to the downlink and
(k+9)th through (k+12)th symbols are allocated to the uplink, as illustrated in FIG. 1. In
the OFDMA frame, a DL/UL MAP burst delivers configuration information (e.g.
position, length, and MCS level) about a plurality of downlink/uplink bursts allocated
to the frame. The bursts other than the DL/UL MAP burst transfer a DL/UL-Medium
Access Control (MAC) layer control message and downlink/uplink data packets. Par-
ticularly, the control message bursts can be a power control change request/command
message burst for controlling the power control scheme of each SS, or a power control
message burst for controlling the transmit power of each SS. The bursts are time-
division-multiple-accessed between SSs and the BS. Transmission gaps called
transmit/receive transition gap (TTG) and receive/transmit transition gap (RTG) are
inserted between the downlink and uplink periods.
[10] Meanwhile, each SS performs initial ranging and periodic ranging to correct time
and frequency errors in uplink bursts and control power. When the SS attempts
ranging, the BS measures the power of a signal from the SS and transmits to the SS a
MAC message including a compensation value for signal power loss caused by path at-
tenuation and rapid signal power change.
[11] Now a description will be made of an uplink power control method in a normal
mode in the OFDM/OFDMA TDD system. The uplink power control is executed in

two steps.
[12] In the first step, the BS carries out power control. The BS scheduler determines
available resources and an available MCS level for uplink transmission within the
transmit power range of an SS of interest by
[13]

where SNRreq and BWreq respectively denote the required SNR and bandwidth for

applying an MCS level to the current packet to be scheduled. SNR UL,RX and BWRX
denote the received SNR and allocated bandwidth of a reference signal, respectively.
The reference signal is a previously received uplink burst signal, a data signal or a
control signal. MARGINTX is a term that represents a channel change. That, this
margin is set considering the difference between the time of scheduling based on
Equation (1) and the actual time of transmitting an uplink signal. Headroom is the
transmit power margin of the SS, calculated by subtracting the current transmit power
from the maximum transmit power of the SS. The BS is assumed to have knowledge of
the maximum transmit power of the SS.

satisfying Equation (1) ensures that the SS transmits an uplink signal with the
resources and MCS level scheduled within the limited power.
[14] In the second step, the SS performs power control. The uplink power control is
considered in two ways: closed-loop power control and open-loop power control.
[15] The uplink closed power control is a scheme of controlling the transmit power of
the SS according to a command from the BS. The BS notifies the SS of a required
power increment/decrement

as well as the resources and MCS level scheduled by Equation (1).
[ 16] The uplink open-loop power control is a scheme of deciding the uplink transmit
power in the SS itself. The BS simply tells the SS the resources and MCS level decided
by Equation (1) and the SS then computes the uplink transmit power of an uplink
signal to be transmitted using the allocated resources by
[17]

where PL and PL denote uplink and downlink path losses, respectively. In view of
the TDD system, these two values are almost the same. The SS can estimate PL
using the transmit power of the BS, PDL.TX and the downlink received power PDL.TX of
°
the SS. NIDL.TX is the power of a signal and interference measured at a receiver of the
BS, common to all of the SSs. SNRreq and BWreq respectively denote the required SNR
and bandwidth for an MCS level to be applied to a packet. MARGIN is a term that
represents the difference between the time to which Equation (2) is computed for ap-
plication and the actual uplink transmission time.
[18] FIG. 2 is a diagram illustrating a signal flow for a conventional closed-loop power
control.
[19] Referring to FIG. 2, the SS transmits a reference signal and information about the
uplink transmit power of the reference signal (UL_Tx, Power) in an uplink burst to the
BS in step 201.
[20] In step 203, the BS (scheduler) calculates the received SNR of the reference signal
and determines resources, an MCS level, and a power increment
AP
for the SS by Equation (1). Headroom involved in Equation (1) can be calculated using
the information of the transmit power (UL_Tx, Power).
[21] In step 205, the BS allocates the uplink resources to the SS according to the
scheduling (UL_MAP) and transmits a power control command (or the power
increment) to the SS. The resource assignment (UL_MAP) information is delivered in
a UL-MAP burst and the power control command is set in a DL burst containing a pre-
determined control message.
[22] The SS determines its uplink transmit power according to the power control
command in step 207 and transmits packets using the allocated resources in step 209.
Thereafter, step 203 (BS scheduling) through step 209 (uplink transmission) are
repeated.
[23] As described before, the power control command is selectively transmitted in the
closed-loop power control. Only if the channel status is changed and the SNR of an
uplink received signal is changed, does the BS transmit a power control command to
the SS. In the absence of the power control command, the SS determines its uplink

transmit power based on the previous uplink transmit power by
[24]

where Pnew and PLast denote the new transmit power and the previous transmit power,

respectively, SNRNew and SNRLast denote a required new SNR and the previous

required SNR, respectively, and BWNew and BWLast denote a new allocated SNR and

the previous allocated SNR, respectively.
[25] FIG. 3 is a diagram illustrating a signal flow for a conventional open-loop power
control.
[26] Referring to FIG. 3, the SS transmits a reference signal and information about the
uplink transmit power of the reference signal (UL_Tx, Power) in an uplink burst to the
BS in step 301.
[27] In step 303, the BS (scheduler) calculates the received SNR of the reference signal
and determines resources, an MCS level, and a power increment

for the SS by Equation (1). Headroom involved in Equation (1) can be calculated using
the information of the transmit power (UL_Tx, Power).
[28] In step 305, the BS allocates the uplink resources to the SS according to the
scheduling (UL_MAP) and transmits the uplink resource assignment (UL_MAP) in-
formation to the SS. Compared to the closed-loop power control, a power control
command is not transmitted in the open-loop power control. Instead, the BS broadcasts
in a DL-MAP burst PDL,TX and NIDL,TX necessary for the computation of Equation (2) to

all of the SSs.
[29] The SS determines its uplink transmit power using the resource assignment in-
formation by Equation (2) in step 307 and transmits an uplink signal using the
allocated resources in step 309. At the same time, the SS tells the BS the current
transmit power. Thereafter, step 303 (BS scheduling) through step 309 (uplink
transmission) are repeated.
[30] As described earlier, in contrast to the closed-loop power control, the open-loop
power control scheme provide to the BS information about the current uplink transmit
power along with the uplink transmission because the SS can change the uplink
transmit power freely. Equation (2) that the SS uses in deciding the transmit power
includes a channel variation which is not known to the BS and thus the headroom of
the SS is changed, unnoticed by the BS. Therefore, the SS tells the BS the current

transmit power at every uplink transmission so that the BS can update the headroom.
[31] On the other hand, in the closed-loop power control, the transmit power of the SS
is changed by a power control command from the BS or a transmit power calculation
formula (Equation (3)) known to the BS. Accordingly, the BS can distinguish a
transmit power change from a channel change in the SNR estimate of an uplink signal.
That is, the BS can execute a power control taking the channel change into account, as
shown in Equation (1). The headroom can also be calculated using the previous
headroom and the previous power control command or using the transmit power of the
SS that the bas station can estimate by Equation (3). Consequently, the SS does not
need to notify the BS of its transmit power at every uplink transmission in the closed-
loop power control.
[32] The features of the two power control schemes are summarized below in Table 1.
[33]

As noted from Table 1, the closed-loop and open-loop power control schemes differ in
uplink/downlink feedback, scheduling margin, and maximum transmit power margin.
The uplink/downlink feedback has been described before. The scheduling margin is
MARGIN in both power control schemes because a scheduling time point coincides
with an actual uplink transmission time in them. The maximum transmit power margin
is defined as the maximum difference between a required transmit power satisfying
SNR at the receiver and an actual transmit power. For the closed-loop power control,
the maximum transmit power margin is MARGIN since the actual transmit power is
decided at scheduling. For the open-loop power control, the actual transmit power is
decided by Equation (2) and thus the maximum transmit power margin is MARGIN
. The scheduling margin leads to resource assignment loss, and the maximum transmit
power margin results in an increase in total system interference.
[34] If the SS moves slowly, the closed-loop power control performs better on the
whole. Because the channel does not change much at a low mobile velocity, the power
control command is not issued frequendy and thus the amount of downlink feedback
information is small. MARGIN affected by the channel variation is also very small.

Also, the scheduling is done and the transmit power is decided according to the actual
uplink channel status, as in Equation (1). Therefore, the uplink power control can be
performed with high reliability.
[35] On the contrary, if the SS moves fast, the open-loop power control outperforms the
closed-loop power control. The channel changes greatly at a high mobile velocity and
thus the number of occurrences of the power control command in the closed-loop
power control is approximately equal to the number of transmit power feedbacks in the
open-loop power control. However, because MARGIN > MARGIN , the closed-
loop power control tracks the channel variation consuming much resources, or cannot
track the channel variation at all. As a result, the closed-loop power control causes
greater interference than the open-loop power control in the case where the SS moves
fast.
Disclosure of Invention
Technical Solution
[36] As described above, the closed-loop and open-loop power control schemes offer
their benefits according to the velocity of the mobile terminal. Nevertheless, con-
ventional systems adopt only one of the two power control schemes. In another case,
the open-loop power control applies to an initial access, and the closed-loop power
control applies thereafter. Thus, the conventional systems do not fully utilize the
advantages of the closed-loop and open-loop power control schemes.
[37] An object of the present invention is to substantially solve at least the above
problems and/or disadvantages and to provide at least the advantages below. Ac-
cordingly, an object of the present invention is to provide an apparatus and method for
adaptively determining a power control scheme according to mobile velocity in a
mobile communication system.
[38] Another object of the present invention is to provide an apparatus and method for
adaptively detennining a power control scheme according to mobile velocity in an
OFDM/OFDMA TDD mobile communication system.
[39] The above objects are achieved by providing an apparatus and method for
adaptively changing an uplink power control scheme according to mobile status in a
TDD mobile communication system.
[40] According to an aspect of the present invention, in a base station in a mobile com-
munication system supporting a plurality of uplink power control schemes, a mobility
estimator generates a mobility index by estimating the velocity of a subscriber station,
and a power controller selects a power control scheme for the uplink of a subscriber
station from among the plurality of power control schemes by comparing the mobility
index with a threshold.

[41] According to another aspect of the present invention, in a subscriber station device
in a mobile communication system supporting a plurality of power control schemes, a
MAC entity extracts, upon receipt of a power control change command message from
a base station, information about a power control scheme requested by the base station
from the power control change command message, and a power controller selects a
power control scheme according to the extracted information received from the MAC
entity and determines the transmit power of an uplink burst according to the selected
power control scheme.
[42] According to a further aspect of the present invention, in a method of determining
an uplink power control scheme in a mobile communication system supporting a
plurality of uplink power control schemes, a base station selects a power control
scheme for the uplink of a subscriber station according to the status of the subscriber
station and transmits to the subscriber station a power control change command
message including information about the selected power control scheme. The
subscriber station extracts, upon receipt of the power control change command
message from the base station, the power control scheme information from the power
control change command message and selects a power control scheme according to the
extracted information.
[43] According to still another aspect of the present invention, in a method of de-
termining an uplink power control scheme in a mobile communication system
supporting a plurality of uplink power control schemes, a subscriber station transmits
to a base station a power control change request message including information about a
requested power control scheme. The base station selects, upon receipt of the power
control change request message, a power control scheme for the uplink of the
subscriber station and transmits a power control change command message including
information about the selected power control scheme to the subscriber station. The
subscriber station extracts, upon receipt of the power control change command
message from the base station, the power control scheme information from the power
control change command message and selects a power control scheme according to the
information extracted by the subscriber station.
[44] According to yet another aspect of the present invention, in a method of de-
termining an uplink power control scheme in a mobile communication system
supporting a plurality of uplink power control schemes, a base station generates a
mobility index by estimating the velocity of a subscriber station, selects a power
control scheme for the uplink of a subscriber station according to the mobility index,
and transmits to the subscriber station a power control change command message
including information about the selected power control scheme, if the selected power
control scheme is different from a previous power control scheme.

[45] According to yet further aspect of the present invention, in a method of de-
termining an uplink power control scheme in a mobile communication system
supporting a plurality of power control schemes, a subscriber station extracts from the
power control change command message, upon receipt of a power control change
command message from a base station, information about a power control scheme
requested by the base station, selects a power control scheme according to the
extracted information, and determines the transmit power of an uplink burst according
to the selected power control scheme.
Description of Drawings
[46] The above and other objects, features and advantages of the present invention will
become more apparent from the following detailed description when taken in
conjunction with the accompanying drawings in which:
[47] FIG. 1 illustrates an OFDMA uplink/downlink frame structure in an IEEE 802.16
OFDMA system;
[48] FIG. 2 is a diagram illustrating a signal flow for a conventional closed-loop power
control;
[49] FIG. 3 is a diagram illustrating a signal flow for a conventional open-loop power
control;
[50] FIG. 4 is a block diagram of a BS in a TDD communication system according to an
embodiment of the present invention;
[51 ] FIG. 5 is a diagram illustrating power control state transitions of the B S depending
on which power control scheme is selected in the TDD communication system
according to an embodiment of the present invention;
[52] FIG. 6 is a flowchart illustrating an operation for determining an uplink power
control scheme in the BS in the TDD communication system according to an
embodiment of the present invention;
[53] FIG. 7 is a block diagram of an SS in the TDD communication system according to
an embodiment of the present invention;
[54] FIG. 8 is a diagram illustrating power control state transition of the SS depending
on which power control scheme is selected in the TDD communication system
according to an embodiment of the present invention;
[55] FIG. 9 is a flowchart illustrating an operation for determining an uplink power
control scheme in the SS in the TDD communication system according to an
embodiment of the present invention;
[56] FIG. 10 is a flowchart illustrating an operation for requesting a power control
change to the BS in the SS in the TDD communication system according to an
embodiment of the present invention; and
[57] FIG. 11 is a diagram illustrating a flow of messages exchanged between the BS and

the SS in the TDD communication system according to an embodiment of the present
invention.
Best Mode
[58] Preferred embodiments of the present invention will be described herein below
with reference to the accompanying drawings. In the following description, well-
known functions or constructions are not described in detail since they would obscure
the invention in unnecessary detail.
[59] The present invention is intended to provide an apparatus and method for selecting
a closed-loop power control scheme or an open-loop power control scheme for uplink
power control according to mobile status in a mobile communication system. While
the present invention will be described in the context of an IEEE 802.16e com-
munication system, for the sake of convenience, the power control scheme changing
method of the present invention is applicable to all other Time Division Duplex (TDD)
communication systems.
[60] FIG. 4 is a block diagram of a BS in a TDD communication system according to an
embodiment of the present invention. The BS includes a MAC entity 401 connected to
a higher layer, a TDD transmission MODEM 403, a TDD reception MODEM 405, a
duplexer 407, an uplink power controller 409, a mobility estimator 411, and a
scheduler 413.
[61] Referring to FIG. 4, the MAC entity 401 receives transmission data from the higher
layer and processes the received data in compliance with the connection protocol of the
TDD transmission MODEM 403. The MAC entity 401 receives data from the TDD
reception MODEM 405, processes the received data in compliance with the connection
protocol of the higher layer, and provides the processed data to the higher layer.
[62] The TDD transmission MODEM 403, which includes a channel encoder, a
modulator, and an RF transmission unit, converts the data received from the MAC
entity 401 to a form suitable for radio transmission. The modulator performs spreading
in a code division multiple access (CDMA) communication system, and OFDM
modulation (IFFT) in an OFDM communication system.
[63] The TDD reception MODEM 405, which includes an RF reception unit, a de-
modulator, and a channel decoder, recovers a radio signal received from the duplexer
407, and provides the recovered signal to the MAC entity 401.
[64] The duplexer 407 provides a signal received in TDD from an antenna (uplink
signal) to the TDD reception MODEM 405, and provides the transmission signal
received from the TDD transmission MODEM 403 (downlink signal) to the antenna.
[65] The scheduler 413 schedules uplink and downlink data transmission according to
data transmission status and the channel statuses of the individual SSs, and orders the
Subscriber Station (SS)s to transmit and receive data as scheduled. In an IEEE 802.16

communication system, for example, the scheduler 413 generates UL-MAP and DL-
MAP as uplink and downlink configuration information, and the MAC entity 401
receives an uplink signal and transmits a downlink signal according to the UL-MAP
and DL-MAP from the scheduler 413.
[66] The mobility estimator 411 determines a mobility index by estimating the mobility
status of an individual SS from a radio signal received from the SS. Many mobility
status estimation algorithms are available and any one of them can be assumed to be
used herein. In accordance with the embodiment of the present invention, a higher
mobility index indicates a higher mobile velocity.
[67] The uplink power controller 409 is responsible for the closed-loop or open-loop
power control. It determines the resources and an MCS level available to each mobile
terminal in a predetermined method (e.g. Equation (1)) and tells the scheduler 413 the
detennined resources and the MCS level. In the case of the closed-loop power control,
the uplink power controller 409 generates a power control command for an individual
SS to the MAC entity 401. The power control schemes have been described in detail
and their description is not provided herein.
[68] In accordance with the present invention, the uplink power controller 409
determines a power control scheme for the SS based on the mobility index received
from the mobility estimator 411. This determination can be made every set time period
or upon receipt from the SS of a power control change request. If the power control
scheme is changed for the SS, the uplink power controller 409 provides to the MAC
entity 401 a power control command for the SS. The MAC entity generates a power
control change command message according to the power control change command
and provides it to the TDD transmission MODEM 403.
[69] FIG. 5 is a diagram illustrating power control state transitions of the BS depending
on which power control scheme is selected in the TDD communication system
according to an embodiment of the present invention.
[70] Referring to FIG. 5, a status variable called power control mode change (PMC) is
used in deciding a power control scheme. If PMC is '0', it indicates selection of the
closed-loop power control. If the PMC is T, it indicates selection of the open-loop
power control.
[71] In the state where PMC=0, if the mobility index received from the mobility
estimator 411 is less than a threshold, the state PMC=0 is kept, as indicated by
reference numeral 505. If the mobility index is greater than the threshold, the state
PMC=0 is transitioned to the state PMC=l, as indicated by reference numeral 511.
Similarly, in the state where PMC=1, if the mobility index is greater than the threshold,
the state PMC=1 is kept, as indicated by reference numeral 509. If the mobility index
is less than the threshold, the state PMC=1 is transitioned to the state PMC=0, as

indicated by reference numeral 507. If the PMC value is changed, this implies that a
different power control scheme from the previous one has been selected. Thus, a power
control change command is transmitted to the SS, notifying the SS of the change of the
power control scheme.
[72] With reference to the state transition diagram of FIG. 5, the operation of the BS
will be described below.
[73] FIG. 6 is a flowchart illustrating an operation for determining in the BS an uplink
power control scheme in the TDD communication system according to an embodiment
of the present invention. As stated before, a decision can be made as to which power
control scheme is to be used at a set time period or upon receipt of a power control
change request from the SS. These two methods can also be used in combination. The
following description is made under the assumption that the decision is made pe-
riodically.
[74] Referring to FIG. 6, the BS determines if a predetermined time period has elapsed
and thus if it is time to set a power control scheme in step 601. If it is time to set a
power control scheme, the mobility estimator 411 compares a calculated mobility
index with the threshold in step 603. In step 605, the BS compares the mobility index
with the threshold. If the mobility index is less than the threshold, the BS sets PMC to
0 in step 607. Since the mobility index being less than the threshold means that the SS
moves slowly, the power control scheme is set to the closed-loop power control. On
the contrary, if the mobility index is greater than the threshold, the BS sets PMC to 1 in
step 609. Since the mobility index being greater than the threshold means that the SS
moves fast, the power control scheme is set to the open-loop power control.
[75] In step 611, the BS determines if the PMC has been toggled by comparing the
power control scheme set currently with the previous power control scheme. If PMC
has not been changed, the BS returns to step 601. If PMC has been changed, the BS
transmits to the SS a power control change command message including information
the changed power control scheme in step 613 and returns to step 601. The detailed
structure of the power control change command message is illustrated below in Table
3.
[76] As described above, the BS decides whether to change the power control scheme
and the SS changes its power control scheme only by the power control change
command received from the BS.
[77] FIG. 7 is a block diagram of the SS in the TDD communication system according
to an embodiment of the present invention.
[78] The SS of the present invention includes a MAC entity 701 connected to a higher
layer, a TDD transmission MODEM 703, a TDD reception MODEM 705, a duplexer
707, a power controller 709, and a mobility estimator 711.

[79] Referring to FIG. 7, the MAC entity 701 receives transmission data from the higher
layer and processes the received data in compliance with the connection protocol of the
TDD transmission MODEM 703. The MAC entity 701 receives data from the TDD
reception MODEM 705, processes the received data in compliance with the connection
protocol of the higher layer, and provides the processed data to the higher layer. The
functions of the MAC entity 701 are performed as commanded by the BS scheduler
413. In the IEEE 802.16 communication system, for example, the scheduler 413
generates UL-MAP and DL-MAP as uplink and downlink configuration information,
and the MAC entity 701 of the SS receives a downlink signal and transmits an uplink
signal according to the DL-MAP and UL-MAP received from the scheduler 413.
[80] The TDD transmission MODEM 703, which includes a channel encoder, a
modulator, and an RF transmission unit, converts the data received from the MAC
entity 701 to a form suitable for radio transmission. Particularly, the TDD transmission
MODEM 703 adjusts the transmit power of the uplink signal according to an uplink
transmit power value received from the power controller 709.
[81] The TDD reception MODEM 705, which includes an RF unit, a demodulator, and
a channel decoder, recovers a radio signal received from the duplexer 707, and
provides the recovered signal to the MAC entity 701. The duplexer 707 provides a
signal received in TDD from an antenna (downlink signal) to the TDD reception
MODEM 705, and provides the transmission signal received from the TDD
transmission MODEM 703 (uplink signal) to the antenna.
[82] The mobility estimator 711 determines a mobility index by estimating the mobility
status of the SS from a radio downlink signal received from the BS, and provides the
mobility index to the power controller 709. Many mobility status estimation algorithms
are available and any one of them can be used herein. In accordance with the
embodiment of the present invention, it is assumed that a higher mobility index
indicates a higher mobile velocity.
[83] The power controller 709 is responsible for the closed-loop or open-loop power
control. For the closed-loop power control, the power controller 709 determines uplink
transmit power according to a power control command received from the BS or by
Equation (3), and provides the uplink power transmit power value to the TDD
transmission MODEM 703. For the open-loop power control, the power controller 709
determines the uplink transmit power by Equation (2) and provides it to the TDD
transmission MODEM 703. In the case of calculating the uplink transmit power by
Equation (2) or Equation (3), information about required bandwidth and SNR is
acquired from the resource assignment information (UL-MAP) received from the BS.
These power control schemes have been described before in detail and their description
is not provided herein.

[84] In accordance with the present invention, the power controller 709 adaptively
selects a power control scheme according to the power control change command
received from the BS. To be more specific, the power control change command
message is provided to the MAC entity 701 through the TDD transmission MODEM
705. The MAC entity 701 extracts a power control change command indicating a
power control scheme from the message. The power controller 709 then selects a
power control scheme according to the power control change command received from
the MAC entity 701.
[85] The power controller 709 can request changing the uplink power control scheme to
the BS. Specifically, the power controller 709 selects a power control scheme
according to the mobility index received from the mobility estimator 711 and if the
selected power control scheme is different from the previous one, the power controller
709 transmits the power control change request to the MAC entity 701. Thus the MAC
entity 701 generates a power control change request message and transmits it to the
BS. In this way, the SS only needs to request the change of a power control scheme
and the BS makes a final decision about the power control scheme.
[86] FIG. 8 is a diagram illustrating power control state transition of the SS depending
on which power control scheme is selected in the TDD communication system
according to an embodiment of the present invention.
[87] Referring to FIG. 8, PMC is used in deciding a power control scheme. If PMC is
'0', it indicates selection of the closed-loop power control. If the PMC is '1', it indicates
selection of the open-loop power control.
[88] In the state where PMC=0, if the power control change command received from the
BS indicates the closed-loop power control, the state PMC=0 (closed-loop power
control) is kept, as indicated by reference numeral 805. If the power control change
command indicates the open-loop power control, the state PMC=0 is transitioned to the
state PMC=1 (open-loop power control), as indicated by reference numeral 811.
Similarly, in the state where PMC=1, if the power control change command indicates
the open-loop power control, the state PMC=1 (open-loop power control) is kept, as
indicated by reference numeral 809. If the power control change command indicates
the closed-loop power control, the state PMC=1 is transitioned to the state PMC=0
(closed-loop power control), as indicated by reference numeral 807. In this way, the SS
determines the power control scheme according to the power control change command
from the BS.
[89] With reference to the state transition diagram of FIG. 8, the operation of the SS
will be described below.
[90] FIG. 9 is a flowchart illustrating an operation for determining an uplink power
control scheme in the SS in the TDD communication system according to an

embodiment of the present invention.
[91] Referring to FIG. 9, the SS determines if a power control change command
message has been received from the BS in step 901. Upon receipt of the power control
change command message, the SS checks in step 903 a power control change
command set in the message. In step 905, the SS determines if the power control
change command indicates the closed-loop power control. If it does, the SS sets PMC
to 0 (closed-loop power control) in step 907 and returns to step 901. If the power
control change command indicates the open-loop power control, the SS sets the PMC
to 1 (open-loop power control) in step 909 and returns to step 901.
[92] FIG. 10 is a flowchart illustrating an operation for requesting a power control
change to the BS in the SS in the TDD communication system according to an
embodiment of the present invention.
[93] Referring to FIG. 10, the SS compares a mobility index calculated by the mobility
estimator 711 with a predetermined threshold in step 1001 and determines if the
mobility index is less than the threshold in step 1003. If the mobility index is less than
the threshold, the SS sets PMC to 0 (closed-loop power control) in step 1005. Since the
mobility index being less than the threshold means that the SS moves slowly, the
power control scheme is set to the closed-loop power control. On the contrary, if the
mobility index is greater than the threshold, the SS sets PMC to 1 (open-loop power
control) in step 1007. Since the mobility index being greater than the threshold means
that the SS moves fast, the power control scheme is set to the open-loop power control.
[94] In step 1009, the SS determines if PMC has been toggled by comparing the power
control scheme set currently (PMC) with the previous power control scheme (PMC). If
PMC has not been changed, the SS returns to step 1001. If PMC has been changed, the
SS transmits to the BS a power control change request message including information
about the changed power control scheme in step 1011 and returns to step 1001. The
detailed structure of the power control change request message is illustrated below in
Table 2.
[95] FIG. 11 is a diagram illustrating a flow of messages exchanged between the BS and
the SS in the TDD communication system according to an embodiment of the present
invention. Particularly, the messages are used in the process of requesting changing by
the SS a power control scheme to the BS and detennining a power control scheme
upon receipt of the power control change request by the BS.
[96] Referring to FIG. 11, when it is necessary to change a power control scheme, the
SS transmits to the BS in step 1101 a power control change request message including
information about a requested power control scheme. The format of the power control
change request message is illustrated below in Table 2.
[97] Upon receipt of the power control change request message, the BS determines a

power control scheme based on the mobility index of the SS in step 1103. If the
determined power control scheme is different from the previous one, the BS transmits
to the SS in step 1105 a power control change command message including in-
formation about the determined power control scheme. The format of the power
control change command message is illustrated below in Table 3.
[98] Upon receipt of the power control change command message, the SS sets in step
1107 a power control scheme according to a power control change command set in the
received message.
[99] As described above, the SS requests the change of a power control scheme and the
BS transmits a power control change command to the SS in response to the power
control change request. In another case, the BS can transmit the power control change
command according to the mobility index to the SS, without receiving the power
control change request. The power control change request message is transmitted to the
BS in a UL burst and the power control change command message is transmitted to the
SS in a DL burst, as illustrated in FIG. 1. Configuration information about the UL burst
and me DL burst are delivered to the SS in a UL-MAP burst and a DL-MAP burst.
That is, the SS transmits the power control change request message and receives the
power control change command message using the MAP information received from
the BS.
[100] Table 2 below illustrates an example of the power control change request message
depicted in FIG. 11, which can be transmitted from the SS in the IEEE 802.16 com-
munication system. It is delivered to the BS in a UL burst.

[101] Referring to Table 2, 'Management Message Type' is a serial number that identifies
the message in the IEEE 802.16 communication system. It can be changed according

to a system standardization. 'Power control mode change' indicates a requested power
control scheme. It is set to '0' for the closed-loop power control and to '1' for the open-
loop power control. 'UL Tx power' indicates the transmit power value of the uplink
burst that delivers the power control change request. Encoding of the transmit power
value is performed in compliance with IEEE 802.16, which will not be described in
detail herein. The BS can utilize the transmit power value for power control, set in the
uplink burst with the power control change request. 'Reserved' represents bits inserted
to match the total size of the message in bytes.
[102] Table 3 below illustrates an example of the power control change command
message depicted in FIG. 11, which can be transmitted from the BS in the IEEE 802.16
communication system. It is delivered to the base mobile in a DL burst.

[103] Referring to Table 3, 'Management Message Type" is a serial number that identifies

the message in the IEEE 802.16 communication system. It can be changed according
to a system standardization. 'Power control mode change' indicates a requested power
control scheme. It is set to '0' for the closed-loop power control and to '1' for the open-
loop power control. 'Start frame' indicates a frame in which the indicated power control
scheme starts to be applied in the IEEE 802.16 communication system. If the indicated
power control scheme is the closed-loop power control, a power control command
'Power adjust' about the transmit power of the SS is transmitted. In the case of the
open-loop power control, an offset value 'OffsetperSS ' is transmitted to be reflected in
MARGINRX of Equation (2). This offset value is specific to the SS, like the change of
link performance caused by channel selectivity and the diversity gain of BS antennas.
In this case, MARGINRX reflects the channel status of the SS as well as the time delay
J
until the power control scheme is applied.
[ 104] Table 4 below illustrates an example of a bandwidth request and uplink transmit
power report message that can be transmitted by the SS in the IEEE 802.16 com-
munication system.

[105] Referring to Table 4, the bandwidth request and uplink transmit power report
message is a modification to an existing IEEE 802.16 bandwidth request message. In
general, uplink communications starts with a bandwidth request from the SS in the
IEEE 802.16 communication system. Thus, the bandwidth request message was
defined in the IEEE 802.16 communication system. Assuming that the uplink commu-
nications start with the bandwidth request from the SS, an uplink message transmitted
from the SS when the procedure illustrated in FIG. 2 or FIG. 3 can be the bandwidth
request message. Yet, this message cannot be used as a reference signal for power
control in the procedure because it does not have information about uplink transmit
power. Accordingly, the SS transmits a bandwidth request and an uplink transmit
power value together in the present invention. In this context, the bandwidth request
and uplink transmit power report message illustrated in Table 4 is designed to serve as
the reference signal for power control. Particularly, this message is in a control
message format called a header according to IEEE 802.16.
[ 106] In Table 4, 'HT (Header Type)' indicates a header type. It is set to ' 1' all the time.

'EC (Encryption Control)' indicates if the payload following the header is encrypted or
not. 'EC is always set to '1'. The bandwidth request and uplink transmit power report
message is configured to have a header only, without payload. 'Type' indicates the type
of the bandwidth request header. It can be changed according to standardization. 'BR'
is short for Bandwidth Request It indicates the amount of uplink data in bytes. 'UL Tx
Power' indicates the transmit power value of a UL burst that carries the bandwidth
request and uplink transmit power report message. Encoding of the transmit power
value performed in compliance with IEEE 802.16, and its description will not be
provided herein. The BS can utilize the transmit power for power control transmit, set
in the uplink burst with the bandwidth request and uplink transmit power report
message. 'CID (Connection ID)' is a 16-bit IEEE 802.16 connection ID. 'HCS (Header
Check Sequence)' is a 8-bit cyclic redundancy check (CRC) value for the message, to
be used for error detection in the BS. The CRC operation is based on IEEE 802.16 and
its description will not be provided herein.
[107] In accordance with the present invention as described above, an uplink power
control scheme is changed in a TDD communication system. Therefore, the uplink
power control can be carried out more efficiently. That is, an efficient uplink power
control is provided by fully utilizing the advantages of the closed-loop and open-loop
power control schemes.
[ 108] While the invention has been shown and described with reference to certain
preferred embodiments thereof, they are merely exemplary applications. For example,
while the closed-loop power control and the open-loop power control have been
described as available power control schemes, the present invention is applicable to
further-divided power control schemes. Therefore, it will be understood by those
skilled in the art that various changes in form and details may be made therein without
departing from the spirit and scope of the invention as defined by the appended claims.

We Claim:
1. A method of determining an uplink power control mode in a
broadband wireless communication system supporting an open loop power
control and a closed loop power control, comprising the steps of:
transmitting a power control change request message from a subscriber
station (SS) to base station (BS) to change the power control mode,
deciding by the BS of the change of the power control mode between the
open loop power control and closed loop power control,
transmitting a power control change response message from the BS to the
SS comprising the decided power control mode;
characterized in that the power control change response message
comprises a power control mode change field indicating the decided power
control mode, a start frame field indicating a frame number at which the
indicated power control mode is activated, a power adjust field indicating a
change in power level that the SS should apply to its current transmission power
if the indicated power control mode is the closed-loop power control mode, and
an offset field indicating a change in power level that the SS should apply to an
open loop power control formula if the indicated power control mode is the open-
loop power control mode.[table 3]

2. A method of determining an uplink power control scheme in a Base Station
(BS) of a wireless communication system supporting at least two uplink power
control schemes, the method comprising:
receiving, from a Subscriber Station (SS), a power control change
request message; [step 1101 of FIG 11] and
transmitting a power control change response message to the SS [step
1105 of FIG 11],
wherein the power control change request message comprises a first
power control mode change field indicating a first power control scheme
requested by the SS, and an UpLink (UL) Transmission (Tx) power field
indicating a transmit power of an uplink burst that carries the power control
change request message [table 2], and
wherein the power control change response message comprises a second
power control mode change field indicating a second power control scheme
decided by the BS, a start frame field indicating a frame number at which the
second power control scheme is activated, a power adjust field indicating a
change in power level that the SS should apply to its current transmission power
if the second power control scheme is a closed-loop power control scheme, and
an offset field indicating a change in power level that the SS should apply to an
open loop power control formula if the second power control scheme is an open-
loop power control scheme [table 3].

3. The method as claimed in claim 2, wherein transmitting the power control
change response message to the SS comprises:
deciding the second power control scheme for an uplink of the SS [step
605 to 609 of FIG 6];
comparing the decided second power control scheme with a previous
power control scheme [step 611 of FIG 6]; and
transmitting the power control change response message to the SS, if the
decided second power control scheme is different from the previous power
control scheme [step 613 of FIG 6].
4. A method of determining an uplink power control scheme in a Subscriber
Station (SS) for a wireless communication system supporting at least two uplink
power control schemes, the method comprising:
transmitting, to a Base Station (BS), a power control change request
message [step 1011 of FIG 10, step 1101 of FIG 11]|; and
receiving a power control change response message from the BS [step
901 of FIG 9, step 1105 of FIG 11],
wherein the power control change request message comprises a first
power control mode change field indicating a first power control scheme
requested by the SS, and an UpLink (UL) Transmission (Tx) power field
indicating a transmit power of an uplink burst that carries the power control
change request message [table 2], and

wherein the power control change response message comprises a second
power control mode change field indicating a second power control scheme
decided by the BS, a start frame field indicating a frame number at which the
second power control scheme is activated, a power adjust field indicating a
change in power level that the SS should apply to its current transmission power
if the second power control scheme is a closed-loop power control scheme, and
an offset field indicating a change in power level that the SS should apply to an
open loop power control formula if the second power control scheme is an open-
loop power control scheme [table 3].
5. The method as claimed in claim 4, comprising determining, by the
subscriber station, whether to change a power control scheme based on the
velocity of the subscriber station.
6. The method as claimed in claim 4, wherein transmitting the power control
change request message to the BS comprises:
deciding the first power control scheme according a status of the SS
[step 1003 to 1007 of FIG 10];
comparing the decided first power control scheme with a previous power
control scheme [step 1009 of FIG. 10]; and
transmitting the power control change request message to the BS, if the
decided first power control scheme is different from the previous power control
scheme [step 1011 of FIG 10].

7. The method as claimed in claim 4, comprising transmitting a bandwidth
request message to the BS, if the SS requests a bandwidth from the BS [below of
table 4],
wherein the bandwidth request message comprises a Bandwidth Request
(BR) field indicating an amount of uplink data to be transmitted and an UL Tx
power field indicating a transmit power of an uplink burst that carries the
bandwidth request message [table 4].
8. A method of determining an uplink power control scheme in a Base Station
(BS) of a wireless communication system supporting at least two uplink power
control schemes, the method comprising:
deciding a power control scheme for an uplink of a Subscriber Station
(SS) [step 605 to 609 of FIG 6]; and
transmitting a power control change response message including
information about the decided power control scheme to the SS [step 613 of FIG
61,
wherein the power control change response message comprises a power
control mode change field indicating the decided power control scheme, a start
frame field indicating a frame number at which the indicated power control
scheme is activated, a power adjust field indicating a change in power level that
the SS should apply to its current transmission power if the indicated power
control scheme is a closed-loop power control scheme, and an offset field
indicating a change in power level that the SS should apply to an open loop

power control formula if the indicated power control scheme is an open-loop
power control scheme [table 3].
9. The method as claimed in claim 8, wherein deciding the power control
scheme is performed, upon request from the SS [step 1103 of FIG 11].
10. The method as claimed in the claim 8, wherein transmitting the power
control change response message to the SS comprises:
comparing the decided power control scheme with a previous power
control scheme [step 611 of FIG 6]; and
transmitting the power control response message to the SS, if the decided
power control scheme is different from the previous power control scheme [step
613 of FIG 6].
11. A method of determining an uplink power control scheme in a Subscriber
Station (SS) of a wireless communication system supporting at least two power
control schemes, the method comprising:
receiving a power control change response message from a Base Station
(BS) [step 901 of FIG 9, step 1105 of FIG 11];
detecting information about a power control scheme decided by the BS
from the power control change response message ;and
determining a transmit power of an uplink signal according to the power
control scheme [step 207 of FIG 2, step 307 of FIG 3],

wherein the power control change response message comprises a power
control mode change field indicating a power control scheme decided by the BS,
a start frame field indicating a frame number at which the indicated power
control scheme is activated, a power adjust field indicating a change in power
level that the SS should apply to its current transmission power if the indicated
power control scheme is a closed-loop power control scheme, and an offset field
indicating a change in power level that the SS should apply to an open loop
power control formula if the indicated power control scheme is an open-loop
power control scheme [table 3].
12. The method as claimed in claim 11, comprising transmitting a bandwidth
request message to the BS, if the SS requests a bandwidth from the BS [below of
table 4],
wherein the bandwidth request message comprises a Bandwidth Request
(BR) field indicating an amount of uplink data to be transmitted and an UpLink
(UL) Transmission (Tx) power field indicating a transmit power of an UL burst
that carries the bandwidth request message [table 41.
13. A broadband wireless communication system supporting an open loop
power control and a closed loop power control, the system comprising:
a Subscriber Station (SS) for transmitting a power control change request
message to a Base Station (BS) to change a power control scheme; and

the BS for deciding to change the power control scheme between the open
loop power control and closed loop power control, and for transmitting, to the SS,
a power control change response message comprising the decided power control
scheme,
wherein the power control change response message comprises a power
control mode change field indicating the decided power control scheme, a start
frame field indicating a frame number at which the indicated power control
scheme is activated, a power adjust field indicating a change in power level that
the SS should apply to its current transmission power if the indicated power
control scheme is the closed-loop power control scheme, and an offset field
indicating a change in power level that the SS should apply to an open loop
power control formula if the indicated power control scheme is the open-loop
power control scheme.
14. An apparatus for a Base Station, BS, in a wireless communication system
supporting at least two uplink power control schemes, the apparatus comprising:
a modem (403, 405) for receiving from a Subscriber Station, SS, a power
control change request message and for transmitting a power control change
response message to the SS,
wherein the power control change request message comprises a first
power control mode change field indicating a first power control scheme
requested by the SS, and an UpLink, UL, Transmission, Tx, power field
indicating a transmit power of an uplink burst that carries the power control

change request message, and
wherein the power control change response message comprises a second
power control mode change field indicating a second power control scheme
decided by the BS, a start frame field indicating a frame number at which the
second power control scheme is activated, a power adjust field indicating a
change in power level that the SS should apply to its current transmission power
if the second power control scheme is a closed-loop power control scheme, and
an offset field indicating a change in power level that the SS should apply to an
open loop power control formula if the second power control scheme is an open-
loop power control scheme.
15. The apparatus as claimed in claim 14, comprising:
a controller (409) for deciding the second power control scheme for an
uplink of the SS, and for comparing the decided second power control scheme
with a previous power control scheme; and
wherein the modem (403, 405) transmits the power control change
response message to the SS, if the decided second power control scheme is
different from the previous power control scheme.
16. An apparatus of determining an uplink power control scheme in a
Subscriber Station, SS, for a wireless communication system supporting at least
two uplink power control schemes, the apparatus comprising:
a modem (703, 705) for transmitting, to a Base Station, BS, a power

control change request message and for receiving, from the BS, a power control
change response message,
wherein the power control change request message comprises a first
power control mode change field indicating a first power control scheme
requested by the SS, an UpLink, UL, Transmission, Tx, power field indicating a
transmit power of an uplink burst that carries the power control change request
message, and
wherein the power control change response message comprises a second
power control mode change field indicating a second power control scheme
decided by the BS, a start frame field indicating a frame number at which the
second power control scheme is activated, a power adjust field indicating a
change in power level that the SS should apply to its current transmission power
if the second power control scheme is a closed-loop power control scheme, and
an offset field indicating a change in power level that the SS should apply to an
open loop power control formula if the second power control scheme is an open-
loop power control scheme.
17. The apparatus as claimed in claim 16, comprising:
a controller (709) for determining whether to change a power control
scheme based on the velocity of the SS.

18. The apparatus as claimed in claim 16, comprising:
a controller (709) for deciding the first power control scheme according a
status of the SS, and for comparing the decided first power control scheme with a
previous power control scheme,
wherein the modem (703, 705) transmits the power control change
request message to the BS, if the decided first power control scheme is different
from the previous power control scheme.
19. The apparatus as claimed in claim 16, wherein the modem (703, 705)
transmits a bandwidth request message to the BS, if the SS requests a bandwidth
from the BS, and
wherein the bandwidth request message includes a Bandwidth Request
(BR) field indicating an amount of uplink data to be transmitted and an UL Tx
power field indicating a transmit power of an uplink burst that carries the
bandwidth request message.
20. An apparatus for a Base Station, BS, in a wireless communication system
supporting at least two uplink power control schemes, the apparatus comprising:
a controller (409) for deciding a power control scheme for an uplink of a
Subscriber Station, SS; and
a modem (403, 405) for transmitting a power control change response
message including information about the decided power control scheme to the SS,

wherein the power control change response message comprises a power
control mode change field indicating the decided power control scheme, a start
frame field indicating a frame number at which the indicated power control
scheme is activated, a power adjust field indicating a change in power level that
the SS should apply to its current transmission power if the indicated power
control scheme is a closed-loop power control scheme, and an offset field
indicating a change in power level that the SS should apply to an open loop
power control formula if the indicated power control scheme is an open-loop
power control scheme.
21. The apparatus as claimed in claim 20, wherein the controller (409) decides
the power control scheme, upon request from the SS.
22. The apparatus as claimed in the claim 20, wherein the controller (409)
compares the decided power control scheme with a previous power control
scheme; and
wherein the modem (403, 405) transmits the power control response
message to the SS, if the decided power control scheme is different from the
previous power control scheme.
23. An apparatus for a Subscriber Station, SS, in a wireless communication
system supporting at least two power control schemes, the apparatus comprising:
a modem (703, 705) for receiving a power control change response

message from a Base Station, BS; and
a controller (709) for determining a transmit power of an uplink signal
according to the power control change response message;
wherein the power control change response message comprises a power
control mode change field indicating a power control scheme decided by the BS,
a start frame field indicating a frame number at which the indicated power
control scheme is activated, a power adjust field indicating a change in power
level that the SS should apply to its current transmission power if the indicated
power control scheme is a closed-loop power control scheme, and an offset field
indicating a change in power level that the SS should apply to an open loop
power control formula if the indicated power control scheme is an open-loop
power control scheme.
24. The apparatus as claimed in claim 23, wherein the modem (703, 705)
transmits a bandwidth request message to the BS, if the SS requests a bandwidth
from the BS;
wherein the bandwidth request message includes a Bandwidth Request,
BR, field indicating an amount of uplink data to be transmitted and an UpLink,
UL, Transmission, Tx, power field indicating a transmit power of an UL burst
that carries the bandwidth request message.

ABSTRACT

TITLE: " A METHOD AND APPARATUS FOR ADAPTIVELY CHANGING UPLINK
POWER CONTROL SCHEME ACCORDING TO MOBILE STATUS IN A TDD MOBILE
COMMUNICATION SYSTEM".
An apparatus and method for changing an uplink power control scheme
according to mobile status in a TDD mobile communication system are
provided. A subscriber station transmits to a base station a power control
change request message including information about a requested power control
scheme. Upon receipt of the power control change request message, the base
station selects a power control scheme for the uplink of the subscriber station
and transmits to the subscriber station a power control change command
message including information about the selected power control scheme. The
subscriber station extracts, upon receipt of the power control change command
message from the base station, the power control scheme information from the
power control change command message and selects a power control scheme
according to the extracted information.